OAuthRest is an implementation of a resourceful REST api. It uses authentication via the oauth-provider plugin.
The XData Toolkit is an XML/XSLT transformation engine and Database Toolkit that uses many sources including MySQL, XML, RSS Feeds and Web Services.
WPoniOS - Rest API is a plugin which provide a rest interface in WordPress CMS.
Salesforce Integration Suite. Web-to-Lead Form & REST API
This plugin is including the Flattr button in your RSS/Atom feed. There are some more options to extend Flattr functionality like optional autosubmit
The WP-RESTful Categories Plugin is a REST API module to be used with WP-RESTful Plugin for WordPress.
The WP-RESTful Tags Plugin is a REST API module to be used with WP-RESTful Plugin for WordPress.
The WP-RESTful User Plugin is a REST API module to be used with WP-RESTful Plugin for WordPress.
This plugin adds a fully customizable sidebar-widget to list an artists upcoming shows or top tracks from Last.FM (using the new 2.0 REST API).
The plugin checks for machine tags with ISBN or ASIN numbers, gets the product data from Amazon, and displays it in the sidebar or in a blog article.